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  <div class="section" id="method-has-no-argument">
<h1>Method has no argument<a class="headerlink" href="#method-has-no-argument" title="Permalink to this headline">¶</a></h1>
<p>Unlike some programming languages, Python does not pass references to instance or class objects automatically behind the scenes. So the program must explicitly pass them as arguments whenever it wants to access any members of the instance or class within a method.</p>
<div class="section" id="anti-pattern">
<h2>Anti-pattern<a class="headerlink" href="#anti-pattern" title="Permalink to this headline">¶</a></h2>
<p>In the <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> class below the <code class="docutils literal notranslate"><span class="pre">area</span></code> method attempts to return the value of the <code class="docutils literal notranslate"><span class="pre">area</span></code> instance variable. However, <code class="docutils literal notranslate"><span class="pre">self.area</span></code> is undefined because a reference to the instance object has not been explicitly passed as an argument to the method.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">=</span> <span class="n">width</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">height</span> <span class="o">=</span> <span class="n">height</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">area</span> <span class="o">=</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
    <span class="c1"># missing first argument &quot;self&quot;</span>
    <span class="k">def</span> <span class="nf">area</span><span class="p">():</span>
        <span class="c1"># self is undefined here</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">area</span>
</pre></div>
</div>
<div class="section" id="class-method-is-missing-the-cls-keyword">
<h3>Class method is missing the <code class="docutils literal notranslate"><span class="pre">cls</span></code> keyword<a class="headerlink" href="#class-method-is-missing-the-cls-keyword" title="Permalink to this headline">¶</a></h3>
<p>The method <code class="docutils literal notranslate"><span class="pre">print_class_name</span></code> attempts to print the name of the class. However, to programmatically access a class name, a method needs to have a reference to the class object. This is accomplished by passing the keyword <code class="docutils literal notranslate"><span class="pre">cls</span></code> as the first argument to the method. Because <code class="docutils literal notranslate"><span class="pre">print_class_name</span></code> does not do this, its reference to <code class="docutils literal notranslate"><span class="pre">cls</span></code> in the body of the method is undefined.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
    <span class="nd">@classmethod</span>
    <span class="c1"># missing first argument &quot;cls&quot;</span>
    <span class="k">def</span> <span class="nf">print_class_name</span><span class="p">():</span>
        <span class="c1"># cls is undefined here</span>
        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Hello, I am </span><span class="si">{0}</span><span class="s2">!&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">cls</span><span class="p">))</span>
</pre></div>
</div>
<p>The method <code class="docutils literal notranslate"><span class="pre">area</span></code> computes the value of any rectangle. Currently this method is ambiguous. It is defined as a method of the <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> class, yet it does not reference any instance or class members. The method needs to explicitly state that it is a static method via the <code class="docutils literal notranslate"><span class="pre">&#64;staticmethod</span></code> decorator.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
    <span class="c1"># &quot;@staticmethod&quot; should be here</span>
    <span class="k">def</span> <span class="nf">area</span><span class="p">(</span><span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
        <span class="k">return</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="best-practices">
<h2>Best practices<a class="headerlink" href="#best-practices" title="Permalink to this headline">¶</a></h2>
<div class="section" id="add-the-self-parameter-to-instance-methods">
<h3>Add the <code class="docutils literal notranslate"><span class="pre">self</span></code> parameter to instance methods<a class="headerlink" href="#add-the-self-parameter-to-instance-methods" title="Permalink to this headline">¶</a></h3>
<p>To access the <code class="docutils literal notranslate"><span class="pre">area</span></code> member of a <code class="docutils literal notranslate"><span class="pre">Rectangle</span></code> instance the first argument of the <code class="docutils literal notranslate"><span class="pre">area</span></code> method needs to be a reference to the instance object, signified by the keyword <code class="docutils literal notranslate"><span class="pre">self</span></code>.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
    <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">width</span> <span class="o">=</span> <span class="n">width</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">height</span> <span class="o">=</span> <span class="n">height</span>
        <span class="bp">self</span><span class="o">.</span><span class="n">area</span> <span class="o">=</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
    <span class="c1"># instance members now accessible because of &quot;self&quot;</span>
    <span class="k">def</span> <span class="nf">area</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
        <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">area</span>
</pre></div>
</div>
</div>
<div class="section" id="add-the-cls-parameter-to-class-methods">
<h3>Add the <code class="docutils literal notranslate"><span class="pre">cls</span></code> parameter to class methods<a class="headerlink" href="#add-the-cls-parameter-to-class-methods" title="Permalink to this headline">¶</a></h3>
<p>To access the name of the class the <code class="docutils literal notranslate"><span class="pre">print_class_name</span></code> method needs to explicitly pass an argument to the class object. This is done by adding the keyword <code class="docutils literal notranslate"><span class="pre">cls</span></code> as the first argument of the method.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
    <span class="nd">@classmethod</span>
    <span class="c1"># class members now accessible, thanks to &quot;cls&quot;</span>
    <span class="k">def</span> <span class="nf">print_class_name</span><span class="p">(</span><span class="bp">cls</span><span class="p">):</span>
        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Hello, I am </span><span class="si">{0}</span><span class="s2">!&quot;</span><span class="o">.</span><span class="n">format</span><span class="p">(</span><span class="bp">cls</span><span class="p">))</span>
</pre></div>
</div>
</div>
<div class="section" id="add-the-staticmethod-decorator-to-static-methods">
<h3>Add the <code class="docutils literal notranslate"><span class="pre">&#64;staticmethod</span></code> decorator to static methods<a class="headerlink" href="#add-the-staticmethod-decorator-to-static-methods" title="Permalink to this headline">¶</a></h3>
<p>If the method is a static method that does not need access to any instance members, then the method should be preceded by the <code class="docutils literal notranslate"><span class="pre">&#64;staticmethod</span></code> decorator. This improves readability by helping clarify that the method should never rely on any instance members.</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Rectangle</span><span class="p">:</span>
    <span class="c1"># clarifies that the method does not need any instance members</span>
    <span class="nd">@staticmethod</span>
    <span class="k">def</span> <span class="nf">area</span><span class="p">(</span><span class="n">width</span><span class="p">,</span> <span class="n">height</span><span class="p">):</span>
        <span class="k">return</span> <span class="n">width</span> <span class="o">*</span> <span class="n">height</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><p>PyLint - E0211, no-method-argument</p></li>
</ul>
</div>
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